"SYLPH" Radiometric Calibration Light Source

A NIST traceable light source to enable radiometric correction of a spectrometer output. Primarily for use with any microscope based spectrometer system

“SYLPH™” Standard Yield Lamp for Photonic Calibration

How the SYLPH radiometric calibration light source works

The Sylph radiometric calibration system setup (not to scale). The illumination head is placed facing the microscope objective

Why calibrating a spectrometer is necessary:

Instrument to instrument variability: Unless they have been “standardized” all spectrometers will characterize a reference emission spectrum differently. This is a function of many issues including variations in detector efficiency, bandpass and various system optics

Instrument dependent spectral resolution: If a natural spectrum has features with a Full Width at Half Maximum (FWHM) that is less than the bandpass (resolution) of the spectrometer then the spectrum will be observed with the spectrometer’s FWHM, not the natural line width.

This is an example of an instrument constraint. The observed bandpass of the sample spectrum is a convolution of the instrumental bandpass and the natural line widths present in the sample spectrum.

For example, if the natural full width at half maximum (FWHM) of a low-pressure mercury (Hg/Ar) lamp is some hundredths of a nanometer (nm), and the bandpass of the spectrometer is set to say 1 nm, then all line widths will be 1 nm, not the natural line width.

Therefore, a Hg/Ar lamp such as the MIDL can be used to determine the actual spectral bandpass and wavelength accuracy of any spectrometer operating in the UV to near IR.

Uncertainties in wavelength ratios: If a sample spectrum covers an extended spectral range, wavelength ratios are very likely to vary between instruments, sometimes significantly. This is due to instrumental component variables. If measuring accurate wavelength ratios is necessary then the data delivered by an instrument needs to be “normalized” to a standard.

Without correction, data will be highly reproducible, yet wavelength ratios will be “precisely inaccurate.” The main wavelength dependent culprits include:

Optical coatings, lenses, mirrors, filters, polarizers and any other optic that is located between the sample and the detector

Aliasing when the system fails to accurately reconstruct an analog spectrum into a digital format.

The impact of all these influences are eliminated when a spectrum is acquired in %transmission, absorption, or %reflection. In these cases, the sample spectrum is divided by a spectrum of the illumination source that has also passed through the same system.

However, in emission spectroscopy including fluorescence, luminescence and Raman there is no illuminant spectrum to divide by. Therefore, in order to accurately reconstruct wavelength ratio intensities, it is necessary to apply a correction factor at each wavelength.

The NIST certified radiometric calibration solution

The accepted way to normalize a spectrometer is to acquire a spectrum of NIST certified characteristics. For wavelength accuracy use an absolute light source such as the MIDL Hg/Ar emission lamp. For wavelength versus intensity use a NIST certified emission lamp such as the “SYLPH” radiometric calibration light source.

The SYLPH radiometrically calibrated light source delivers a stable, NIST-certified spectrum of known characteristics. This provides hyperspectral microscope and spectral confocal systems with a certified traceable “reference sample” when accurate comparisons are a necessity.

To standardize a system determine a correction curve by acquiring a spectrum of the SYLPH lamp and compare it to the spectrum described in the certificate. After applying the correction curve the spectrometer will reconstruct the spectrum of the SYLPH lamp and accurately reconsruct the spectrum of any other sample..

Once this correction factor has been incorporated in the data processing software it should be applied to all future acquisitions.

Bottom line: All radiometrically calibrated instruments will accurately reproduce the same reference spectrum. Once an instrument has been radiometrically corrected all spectra acquired on that instrument can be compared with those acquired on other radiometrically calibrated instrument.